3.1 Selection of components for the three-way catalysts
3.1.1 Study the complete oxidation of hydrocarbon
As seen from literature review, many catalysts were investigated for hydrocarbon treatment. However, there are not yet agreement about the composition of the best catalyst.
Therefore, some samples were studied systematically in order to find potential catalysts that exhibit the highest activity.
The catalytic properties of MnOx-based catalysts are attributed to the ability of manganese to form oxides of different oxidation states and to their high oxygen storage capacity (OSC) [36]. It can be seen that, Co3O4 resulted in high propylene consumption but low CO2 selectivity [106]. Therefore, in this study, mixtures between MnO2 and Co3O4 were investigated. Chemical mixtures of CeO2 and Co3O4 were also studied since the results in [106] showed that Co3O4 exhibited high propylene conversion at low temperature although it did not exhibit high CO2 selectivity at high temperatures. Meanwhile, CeO2 exhibited high CO2 selectivity at high temperatures although its propylene conversion was not as high as that of Co3O4 at low temperatures. Therefore, when CeO2 and Co3O4 were mixed together, the obtained catalysts may exhibit high conversion of propylene at low temperatures and high CO2 selectivity at high temperatures. The catalyst were firstly tested in oxygen deficient condition because it was believed that if a catalyst is good in O2
deficient condition, it may be better in O2 sufficient condition.
Catalytic activities of the bi-metallic oxides were determined based on both the propylene conversion and the CO2 selectivity at temperature ranges from 200 oC to 500 oC (figure A1, A2, A4, A5 - annex 1.1). Experiment data showed that amongst investigated samples with the content of one oxide component altered from 10 – 90%, the samples MnCo 1-3, MnCo 7-3 and CeCo 1-4 possess the highest C3H6 conversion at all temperatures. The catalytic activity of these samples were presented in Figure 3.1. The results showed that the activity of these mixtures were significantly higher than that of the pure components MnO2, Co3O4, CeO2. This was in accordance with the S.Todorova’s results [36] when studying some mixtures of Mn-Co oxides. The catalytic activity of both single component cobalt and manganese was similar, however, a combination of these oxides (the molar atomic ratio Mn/Co of 5/15 and 15/5) change significantly its activity.
As seen in the literature [80], MnO2 exhibited high oxygen storage capacity, fast oxygen adsorption and oxide reduction rate meanwhile Co3O4 possessed high active electrophilic oxygen for hydrocarbon oxidation. Thus, some MnO2-Co3O4 samples possess high activity of both single components due to the combination of two oxides at favorable molar ratios.
metal oxides for the treatment of exhaust gases from internal combustion engine
0 5 10 15 20 25 30 35 40 45 50
200 250 300 350 400 450 500
Reaction temperature, oC C3H6 conversion, %
MnO2 Co3O4 CeO2 MnCo 1-3 MnCo 7-3 CeCo 1-4
0 20 40 60 80 100
250 300 350 400 450 500
Reaction temperature, oC
CO2selectivity, %
MnO2 Co3O4 CeO2 MnCo 1-3 MnCo 7-3 CeCo 1-4
Figure 3.1 Catalytic activity of some mixed oxide MnCo, CoCe and single metallic oxide in deficient oxygen condition
Since exhibited good activity under deficient oxygen condition, MnCo 1-3 and CeCo 1- 4 catalysts were chosen to test for activity under excess O2 condition. It was clear from the Figure 3.2 that propylene conversion was 89.26% at 250 oC and 100% from 300 oC on MnCo 1-3 catalyst, 87% from 250 oC on CeCo 1-4 catalyst. The catalyst catalyzed essentially the formation carbon dioxide. CO2 selectivity was quite stable at all temperatures with value of approximate 90%. Meanwhile CO selectivity was very low (<10%) and oxygenated product was formed insignificantly under excess oxygen condition.
0 20 40 60 80 100
200 250 300 350 400 450 500
Reaction temperature , oC C3H6 conversion, %
CeCo 1-4 M nCo 1-3
Figure 3.2 Catalytic activity of MnCo 1-3 and CeCo 1-4 catalysts in excess oxygen condition
metal oxides for the treatment of exhaust gases from internal combustion engine
Figure 3.2 shows that activity of CoCe mixture was lower than that of MnCo mixture.
However, from Figure 3.3 activity of this mixture could be improved by increase O2/C3H6
ratio. In this case, the presence of CO didn’t decrease the activity of the catalyst (condition a). The presence of H2O, nevertheless decrease a little activity of this catalyst since the catalyst only obtained 100% conversion from 250 oC instead of 200 oC as in the case of without H2O (condition b).
0 20 40 60 80 100
200 250 300 350 400 450 500
Temperature, oC C3H6 conversion,%
a b
Figure 3.3 C3H6 conversion of CeCo1-4 in different reaction conditions (condition a: excess oxygen condition with the presence of CO: 0.9% C3H6, 0.3% CO, 5% O2, N2 balance, condition b: excess oxygen condition with
the presence of CO and H2O: 0.9% C3H6, 0.3% CO, 2% H2O, 5% O2, N2 balance)
70 60
50 40
30 20
2 theta, degrees
CeCo 1-4
CeO2 Co3O4 CeO2
CeO2 CeO2 CeO2
Co3O4
Co3O4 Co3O4 Co3O4 Co3O4
Co3O4
a)
70 60
50 40
30 20
2 theta, degrees
Co3O4 Co3O4
Co3O4 Co3O4 Co3O4 Co3O4
MnCo 1-3 MnO2
MnO2 MnO2 MnO2 MnO2
b)
Figure 3.4 XRD patterns of CeCo=1-4, MnCo=1-3 chemical mixtures and some pure single oxides With the aim to explain for the good activity of Mn-Co and Ce-Co mixtures, several characterizations were performed. XRD pattern of CeO2-Co3O4 chemical mixtures (CeCo=1-4) showed the presence of CeO2 which was more recognized and Co3O4 phase
metal oxides for the treatment of exhaust gases from internal combustion engine
which was less recognized due to the high amorphous nature of Co3O4. The result was also obtained by M. Dhakad when studying CeO2-Co3O4 synthesized by co-precipitation [125].
Instead, the peaks belonged to CeO2 phase slightly shifted to the higher 2θ value, indicating a formation of solid solution of CeO2 and Co3O4, in which Ce3+ replaced for Co3+ in its structure (Figure 3.4a). Due to the change in structure, the CeO2-Co3O4
chemical mixtures possessed surface areas around 45 m2/g, which were higher than those of pure CeO2 (33 m2/g) and pure Co3O4 (11 m2/g). Similarly, XRD pattern of MnCo 1-3 only showed the peaks belongs to Co3O4, meanwhile, MnO2 peaks couldn’t be seen. The peaks of Co3O4 slightly shifted to the lower 2θ value that indicated the formation of MnO2- Co3O4 solid solution in which manganese ion replaced for cobalt ion in structure (Figure 3.4b). This may be one of the reasons for the higher activity of the mixtures compared to pure components.
For a clearer explanation, TPR-H2 profiles of pure and mixed oxides were investigated.
TPR - H2 of these samples in Table 3.1 showed that Co3O4 exhibited an excellent mobility of oxygen as it consumed the highest H2 quantity amongst the investigated catalysts.
Co3O4 was also reduced at lower temperatures than CeO2, which explained for the fact that Co3O4 exhibited good activity at lower temperature than CeO2. The chemical mixtures of CeCo 1-4 and MnCo 1-3 did not possess a larger quantity of mobile oxygen than pure Co3O4 but was reduced at lower temperature (279 oC and 352.8 oC, respectively), therefore, the chemical mixture of CeCo 1-4 and MnCo 1-3 was able to convert propylene at lower temperature than Co3O4 and the particles diameter may be nanometer (as seen in SEM images below).
Table 3.1 Quantity of hydrogen consumed volume (ml/g) at different reduction peaks in TPR-H2 profiles of pure CeO2, Co3O4, MnO2 and CeO2-Co3O4, MnO2-Co3O4 chemical mixtures
Temperature at maximum of reduction peak
CeO2
(33 m2/g)
Co3O4
(11 m2/g)
MnO2 (5.61 m2/g)
CeCo 1-4 (45 m2/g)
MnCo 1-3 (13.94 m2/g)
279 28.97
339.6 27.05
352.8 26.33
364 12.21
430 250.54
434.1 113.14
474 4.62
479.6 231.27
503 101.25
580 39.25
694 6.23
Total of H2
consumed volume
10.85 289.79 140.19 142.43 257.6
3.1.1.2 Triple metallic oxides
As seen from Figure 3.2, MnCo 1-3 and CeCo 1-4 exhibited good activity for hydrocarbon oxidation. Furthermore, as seen in literature, MnO2-Co3O4-CeO2catalyst with 25% wt.Co3O4/Mn0.9Ce0.1O2 [107] and the atomic ratio of Mn:Co:Ce=1:8:1 [108]
possessed very high oxidation property. These catalysts could convert CO completely under rich H2 condition at low temperature. Therefore, MnO2-Co3O4-CeO2 chemical mixture with the molar ratio of MnCoCe 1-3-0.75, in which the optimal MnO2/Co3O4 ratio and CeO /Co O ratio obtained from the previous section were mostly maintained, was
metal oxides for the treatment of exhaust gases from internal combustion engine
studied for the oxidation of unsaturated hydrocarbon C3H6, saturated hydrocarbon C3H8 and aromatic hydrocarbon C6H6 under sufficient oxygen condition (Figure 3.5).
From Figure 3.5, it can be seen that C3H6 was easier oxidized than C3H8 with conversion of 96.07% and 98.01% at 200 and 250 oC, respectively. At higher temperatures (from 300 oC), the conversion of C3H8 and C3H6 was approximately the same. The catalyst also exhibited superior activity for complete oxidation of C6H6 when reaching maximum conversion from 300 oC. Compared to MnCo 1-3 and CeCo 1-4 and especially single MnO2, Co3O4, CeO2, hydrocarbon conversion on MnCoCe 1-3-0.75 was much higher.
Characterization of single metallic oxides (MnO2, Co3O4, and CeO2) and triple oxides MnO2-Co3O4-CeO2 were performed to explain the phenomenon.
0 20 40 60 80 100
200 250 300 350 400 450 500
Reaction temperature, oC
Conversion of Hydrocarbon, %
C3H6 C3H8 C6H6 C3H6
C6H6
C3H8
Figure 3.5 Conversion of C3H6, C3H8 and C6H6 on MnCoCe 1-3-0.75 catalyst under sufficient oxygen condition Figure 3.6 showed the SEM images of MnCo 1-3 fresh, MnCoCe 1-3-0.75 before and after C3H8 oxidation reaction. The particle diameters of MnCo 1-3 and triple oxides were approximate 30 nm, 10-15 nm, respectively. The smaller particle size can cause the higher activity of MnCoCe compared to that of bi metallic oxide (MnCo 1-3).After reaction, particle size of MnCoCe did not change significantly but there is a trend of sintering. This is due to the high temperature of the reaction and exothermic phenomena of complete oxidation reaction.
a b c
Figure 3.6 SEM images of MnCo 1-3 fresh (a),MnCoCe 1-3-0.75 before (a) and after (b) reaction under sufficient oxygen condition (O2/C3H8=5/1)
Figure 3.7 presented XRD pattern of fresh MnCoCe 1-3-0.75 sample and the original oxides (MnO2, Co3O4, CeO2). The mixed catalyst only exhibited the highest reflection of Co3O4 at 2θ= 31.2o, 36.6o, 44.6o, 59o and 65o due to the highest content of this oxides. No XRD peaks belonged to MnO2, CeO2 can be detected. Compared to XRD patterns of mixture of CeO2, Co3O4, MnO2 (Figure 3.4), it could be seen that with the presence of MnO2, the replace of Co for Ce in structure of CeO2 was disappeared. Instead, a solid
metal oxides for the treatment of exhaust gases from internal combustion engine
solution in which other cations (manganese and cerium) replaced for cobalt in the structure of Co3O4 can be formed from these oxides, as there is a shift of Co3O4 reflection to lower value.
400
300
200
100
0
70 60
50 40
30 20
2 theta, degrees
MnO2 CeO2
Co3O4 CeO2
CeO2 CeO2 CeO2
MnO2
MnO2 MnO2 MnO2
Co3O4 Co3O4 Co3O4 Co3O4
Co3O4
Co3O4
Co3O4 Co3O4 MnCoCe 1-3-0.75